Spark plug with a semiconductor teaser gap



Jan.. 6, 1970 H. A. BURLEY 'ET AL 3,488,544

SPARK 'PLUG WITH A4 SEMICONDUCTOR TEASER GAP Filed Dec. l, 196'? INVENTORS United States Patent O 3,488,544 SPARK PLUG WITH A SEMICONDUCTOR TEASER GAP Harvey A. Burley, Warren, and Richard E. Massoll,

Flint, Mich., assignors to General Motors `Corporation, Detroit, Mich., a corporation of Delaware Filed Dec. 1, 1967, Ser. No. 687,165 Int. CL H01t 13/20 U.S. Cl. 313-131 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to spark plugs, and more particularly to semiconductor spark plugs.

Igniters containing semiconductor materials therein have been employed in gas turbines for some period of time. Turbine igniters are pulsed or sparked perhaps once each second during start-up and at other critical times with relatively large energy discharges in the range of from 1 to 20 joules. This large energy discharge is required to initiate combustion in gas turbine engines because, (l) the fuel-air mixture is in motion, (2) jet fuels do not atomize as readily as gasoline and (3) the igniter is located in the fringe area of the fuel spray pattern.

The requirements of a semiconductor spark plug used in a reciprocating engine differ significantly from those for semiconductor igniters used in gas turbines. A spark plug for a reciprocating engine is pulsed or sparked continuously at a rate of, for example, 50 pulses per second while the engine is operating at 6,000 r.p.m. The energy of the pulse or spark in a reciprocating engine is about 0.05 to 0.08 joule while as little as 0.003 joule will initiate combustion under ideal conditions.

Semiconductor spark plugs which have incorporated the conventional igniter semiconductor-electrode design have heretofore limited success in reciprocating engines. One problem encountered with semiconductor spark plugs is that these spark plugs did not properly ignite the mixture under all engine operating conditions. This occurred primarily because the low energy spark tended to follow the length of the semiconductor surface and was subjected to the quenching eiect of the surface. This problem of not igniting the fuel mixture does not occur in semiconductor igniters because the spark energy is high enough to overcome any quenching effect. Another problem encountered with semiconductor spark plugs is the erosion of the semiconductor which is caused by the continuous arcing along the surface of the semiconductor while the engine is in operation.

A number of potential advantages are anticipated by the use of semiconductor spark plugs in reciprocating engines. One advantage is the reduced problems encountered with electrical insulation since semiconductor ignition systems employ about 2,000 to 10,000 volts compared with conventional spark plug systems which employ up to about 32,000 volts. A second advantage is the nonfouling characteristics due to the tendency of this low impedance spark plug and ignition system to be effected to a lesser degree by deposits between the electrodes which tend to lower the impedance. A third advantage'is that a single design for a semiconductor spark plug is suitable for use over a broad temperature range. Another advan- 3,488,544 Patented Jan. 6, 1970 ICC tage is that semiconductor spark plugs have reduced radio frequency interference.

It is an object of this invention to provide an improved semiconductor spark plug for reciprocating engines. It is another object of this invention to provide a semiconductor spark plug having low semiconductor erosion. It is still another object of this invention to provide a semiconductor spark plug which will operate and ignite fuel mixtures under substantially all engine operating condtions.

These and other objects are accomplished by a semiconductor spark plug having two spark gaps. In addition to the conventional main spark plug gap between the center electrode and the ground electrode, a small or trigger gap is formed between the center electrode and the semiconductor. Voltage impressed upon the semiconductor spark plug initiates arcing across the trigger gap which continues for a time suiiicient to ionize the gaseous molecules in the main gap region thereby reducing the breakdown voltage of the main gap, that is, the voltage required to cause a spark to jump across the main gap between the center electrode and ground electrode. The sparking and/or arcing in this semiconductor plug is limited to the two spark gaps and as a result, sparks and/or arcs do not pass across or over the entire semiconductor surface thereby eliminating the problems inherent in semiconductor spark plugs which utilize a semiconductor igniter design.

Other objects and advantages of this invention will be apparent from the following detailed description, reference being made to the accompanying drawings wherein a preferred embodiment of this invention is shown.

In the drawings:

FIGURE 1 is a side View of an improved semiconductor spark plug; and

FIGURE 2 is an enlarged view of the tiring tip portion of the improved semiconductor spark plug.

Referring now to the drawings, the spark plug 10 comprises a conventional outer metal shell 12 having a ground electrode 14 welded to the lower end thereof. Positioned within the metal shell 12 and secured in the conventional manner is an insulator 16. The insulator 16 should preferably be of a high alumina base material containing upwards of aluminum oxide such, for example, as covered by United States Patent No. 2,760,875 issued to Karl Schwartzwalder and Helen Blair Bartlett. The insulator 16 is formed with a centerbore 18.

Positioned within the lower end of the metal shell 12 is a semiconductor 20 which abuts the lower end 21 of the insulator 16. The semiconductor 20 is formed with a centerbore 22 which is concentric with and adjacent to the insulator centerbore 18. The semiconductor should have a resistivity of the order of to Il()6 ohms at 3,000 volts since lower resistivity values would result in too high an energy loss within the semiconductor at the time voltage is applied. The bulk breakdown value and the surface breakdown value of the semiconductor material should be over 20 kv. per centimeter since lower values would lead to early destruction of the semiconductor and its surface. Semiconductor materials presently being used in low voltage igniters may be used in this invention if they meet the requirements outlined above. The patent to Edwards, No. 3,052,814, describes in detail a silicon nitride-bonded silicon carbide semiconductor which performs satisfactorily. In addition to semiconductor materials, it is also possible to use resistor materials of the type described in the McDougal et al. Patent No. 2,459,282 and the type described in the patents to Counts et al., No. 2,664,884, and No. 2,235,655, or a combination of resistor-semiconductor materials as long as they fulfill the requirements outlined above.

Positioned within the insulator centerbore 18 and within the semiconductor centerbore 22 is the center electrode 24. A tubular insulator 26 is positioned around the center electrode 24 in the clearance space between the center electrode 24 and the walls of the insulator centerbore and the semiconductor centerbore 22. The insulator 26 should preferably be of a high alumina base material containing upwards of 85% aluminum oxide of the type described previously for the insulator 16. The insulator 26 electrically insulates the center electrode 24 from the semiconductor 20. A glass mass 27 hermetically seals the center electrode 24 to the walls of the insulator centerbore 18. The lower end 29 of the insulator 26 is preferably flush with the center electrode firing tip end 28 and the semiconductor firing tip end surface 30. The center electrode firing tip end 28 and the semiconductor surface 30 adjacent the insulator end 29 form a small or trigger gap therebetween. The length of the trigger gap existing between the semiconductor surface 30 and the center electrode firing tip end 28 is from about 0.001 inch to 0.020 inch with the prefered length being 0.003 to 0.010 inch. The length of the trigger gap is usually selected on the basis of performance under adverse conditions such as fouling, flood start, and cold start. The trigger gap is an essential feature of this invention since arcing across this trigger gap reduces the voltage required for sparking across the main spark gap.

The length of the semiconductor surface between the center electrode and the shell should be greater than the length of the main gap between the center electrode and the ground electrode. By having the length of the semiconductor surface greater than the length of the main gap, and assuming the semiconductor has sufficient resistivity, the spark will jump across the main gap rather than follow the surface of the semiconductor. As a result, the surface of the semiconductor is not subject to the erosion it would encounter if the spark had continuously passed across its surface. The spark plug life is increased because of the reduced semiconductor surface erosion. Another beneficial result obtained by not having the spark pass over or across the surface of the semiconductor is that the quenching effect of the semiconductor surface on the small flame kernel is substantially eliminated thereby increasing the effectiveness of igniting the combustible fuel-gas mixture.

The center electrode firing tip end portion 28 and the firing tip end portion 32 of ground electrode 14 form the conventional spark plug main gap. The main gap is about 0.035 inch in length. The length of the main gap depends upon the particular engine in which it is used. As it is well known in the art, a small gap is more efficient for igniting good fuel-air mixtures whereas a large gap is more efficient for igniting lean mixtures and rich mixtures.

The semiconductor spark plug described above is connected in an ignition system which is a modification of the conventional ignition system. The conventional ignition system containing a coil having a 100 to 1 turn ratio and delivering 28 kv. is modified by using a coil having a 30 to 1 turn ratio which in turn delivers about 8 kv.

The ignition system delivers the voltage to the spark plug initiating arcing across the small or trigger gap which continues for a time sufcient to ionize some of the gas in the main gap. The ionization of the gas in the main gap reduces the voltage required to cause a spark to jump across the main gap. In conventional spark plugs it is necessary to deliver up to 24 to 28 kv. to cause a spark to jump across the main gap. The semiconductor spark plug of thisinvention will, due to the ionization of the gas in the main gap caused by the arcing in the trigger gap, spark across the main gap with a voltage ranging from about 2 to 6 or 7 kv. depending upon the operating conditions and the pressure in the combustion chamber.

The improved semiconductor spark plug of this invention has substantially eliminated the spark traversing the length of the semiconductor surface thereby reducing semiconductor erosion and the semiconductor fiame quenching effect which in turn increases spark plug life and increases the effectiveness in igniting fuel-air mixtures.

While the invention has been described in terms of a preferred embodiment, it is to be understood that the scope of the invention is not limited thereby except as defined in the following claims.

What is claimed is:

1. A spark plug comprising a metal shell having a ground electrode secured to one end thereof, a body having a resistivity of to 106 ohm/centimeters at 3,000 volts and having a centerbore therethrough secured Within the ground electrode end of said shell, a center electrode positioned in said centerbore and electrically insulated from said body and said ground electrode, said center electrode having a firing tip end adapted to form a first spark gap with said body and a second spark gap with said ground electrode wherein a voltage impressed upon said spark plug initiates arcing in said first gap which continues for a time Sufiicient to ionize gas in said second gap thereby reducing the voltage required to initiate sparking in said second gap.

2. A semiconductor spark plug comprising a metal shell having a ground electrode secured to one end thereof, a semiconductor having a centerbore therethrough secured within the ground electrode end of said shell, a center electrode positioned in said semiconductor centerbore and electrically insulated from said semiconductor and said ground electrode, said center electrode having a firing tip end adapted to form a first spark gap with said semiconductor and a second spark gap with said ground electrode wherein a voltage impressed upon Said spark plug initiates arcing in said first gap which continues for a time sufficient to ionize gas in said second gap thereby reducing the voltage required to initiate sparking in said second gap.

3. A semiconductor spark plug comprising a metal shell having a ground electrode secured to one end thereof, a semiconductor having the centerbore therethrough secured within the ground electrode end of said shell, a tubular insulator having a centerbore therethrough positioned in said semiconductor centerbore, a center electrode positioned in said insulator centerbore, said center electrode having a firing tip end adapted to form a first spark gap with said semiconductor and a second spark gap with said ground electrode wherein a voltage impressed upon said spark plug initiates arcing in said first gap which continues for a time sufficient to ionize gaS in said second gap thereby reducing the voltage required to initiate sparking in said second gap.

4. A spark plug as described in claim 3 wherein Said first gap is 0.001 inch to 0.020 inch.

5. A spark plug as described in claim 3 wherein said semiconductor has a resistivity of 105 to 106 ohm/centimeters at 3,000 volts.

6. A spark plug as described in claim 3 wherein the length of said semiconductor surface between said Shell and said tubular insulator is greater than the length Of said second spark gap.

7. A spark plug as described in claim 3 wherein said center electrode firing tip e'nd and said tubular insulator end and said semiconductor surface form a flush surface where they meet.

References Cited UNITED STATES PATENTS 2,890,365 6/1959 Major 313-131 X 2,963,620 12/1960 Knudson et al. 313-131 X 3,004,184 10/1961 Deans 313-131 X 3,025,425 3/1962 Logan 313-131 X 3,146,301 8/1964 Logan 313-131 X 3,344,304 9/1967 Rademacher 313-131 JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner U.S. Cl. X.R. 313- PCi-1050 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No' 3,488,544 Dated January 6, 1970 Invented@ Harvey A. Burley and Richard E. Massoll It is certified that error appears in the above-identified patent and that sadLetters Patent are hereby corrected as show-n below:

Column 2, line 69, delete "2,664,884" and "2,235,655" and substitute therefor 2,864,884 and 3,235,655 respectively.

SIGNED AND SEALED JUL 141970 (SEAL) Attest:

Edward M. Flewher, Ir. wxmxm n. mmm, m. nagging Offir Cmmnissioner of Patents 

